The present invention relates to the steering of automobiles.
Variation in road surface friction and profile, asymmetric brake torque and differential acceleration of the front wheels cause disturbances in the longitudinal tyre forces, lateral tyre forces, tyre self-aligning moment and suspension deflection forces. These disturbances are fed to the driver by the steering column affecting subjective evaluation, and in the worst case vehicle instability.
The lateral force on a vehicle's steering rack is a function of the sum of the moments about its left and right kingpin axes. Kingpin moments are generated by longitudinal and lateral tyre forces, suspension deflection forces and tyre self-aligning moments. The force in the steering rack is transmitted to the driver by the steering column.
There is a combination of mean longitudinal and lateral tyre forces, suspension deflection and tyre self-aligning moments with which a vehicle can follow a desired path. However the actual forces and moments fluctuate about the mean values due to road surface friction and profile variation, and asymmetric brake torque applications. This fluctuation can be considered to be a disturbance force applied to the steering system.
Disturbance forces in the steering system will be transmitted to the driver through the steering column, potentially adversely affecting the subjective evaluation of a vehicle. Unexpected disturbances can also dangerously affect the vehicle's stability. Designing steering and suspension geometry with the constraint to minimise the transmission of such disturbances may lead to compromises in other aspects of their design.
Thus it is desirable to be able to substantially reduce such disturbance forces through other means.
Increasing numbers of vehicles are now being fitted with Vehicle Stability Control (VSC) and Electric Power Steering (EPS) systems. VSC regulates a vehicle's motion at the limits of lateral traction through asymmetric brake activation. Its control relies on measured or estimated signals such as yaw rate, lateral acceleration, engine torque, steering angle, vehicle speed, brake pressures and wheel speeds. EPS aids a driver to steer a vehicle by applying a torque to its steering system. It is capable of applying a torque independent of the driver's steering input, although if it is to aid the driver the two are generally linked. EPS uses measured or estimated signals such as steering column angle, velocity and torque, steering rack force and vehicle speed.